The invention relates to improvements in composite sheets in general (hereinafter called foils for short), and more particularly to improvements in foils wherein at least one side of a substrate (e.g., a strip or web of plastic material) is coated with a metallic film, such as a film of aluminum. The invention also relates to a method of making foils which contain metallic films.
Foils of the above outlined character are in widespread use in a number of industries, particularly for the packing or wrapping of foodstuffs and many other products. An advantage of foils which consist of or include metallized substrates is that they enhance the appearance of the packed or wrapped products. Another advantage of such foils is that their initial as well as processing cost is but a fraction of the cost of foils which are made solely of a metallic material, such as aluminum. A further important advantage of foils wherein a usually nonmetallic substrate carries one or more films of metallic material is that they can be provided with one or more additional coats in a simple, time-saving and inexpensive manner.
A drawback of presently known foils wherein a substrate carries one or more films of metallic material is that they are not suitable for a number of important uses. For example, foodstuffs of many kinds, as well as many cosmetic substances, must be packaged or wrapped in such a way that the contents of the package are reliably sealed from the surrounding air as well as from moisture. Heretofore known attempts to produce foils which satisfy such requirements include the utilization of plastic substrates which are laminated with films of a metallic material, such as, aluminum. It is also known to coat metallized plastic foils with layers of polyvinylidene chloride. A drawback of such foils is that they are expensive as well as that they do not satisfy the exacting requirements of environmental protection agencies in many countries. Another drawback of such foils is that they are not fluidtight, or are not sufficiently fluidtight, for a number of purposes. Moreover, their ability to prevent the penetration of gaseous and/or liquid fluids is not predictable and often varies from area to area. While the inability of such conventional foils to prevent the passage of certain gases and/or vaporized liquids might not be detrimental for a number of uses, their permeability to oxygen and/or vapors (such as water vapors) renders them unacceptable for a number of important applications in the food processing and many other industries. Extensive and costly research in this field has so far failed to provide a solution which would broaden the field of application of metallized foils. Moreover, the permeability of conventional foils to oxygen and to certain other fluids increases drastically if their metallic films are permitted to rub against each other and/or against the substrates and/or against the confined product or products.
An object of the invention is to provide a foil which is not permeable to fluids.
Another object of the invention is to provide a foil which can be put to use as any conventional foil and is further suitable for a number of additional important uses.
A further object of the invention is to provide a foil which meets the requirements of environmental protection agencies and which can be utilized with advantage for the wrapping or packing or other confinement of all kinds of foodstuffs, cosmetics and many other products that should be sealed from the surrounding atmosphere.
An additional object of the invention is to provide a foil which is formed in such a way that its metallic layer or layers are reliably protected from scuffing, corrosion and/or other undesirable influences.
Still another object of the invention is to provide a foil which can be stored in convoluted form without any damage to its metallic film or films.
A further object of the invention is to provide a foil which can be made of readily available materials, at a low cost and in readily available machinery.
Another object of the invention is to provide a foil which is more versatile and safer than heretofore known foils even though its thickness need not exceed, and can be less than, the thickness of conventional foils.
An additional object of the invention is to provide a foil which can readily accept and retain printed matter.
Still another object of the invention is to provide a novel and improved method of making the above outlined foil.
A further object of the invention is to provide a method which renders it possible to reinforce and/or otherwise improve the appearance and/or other desirable characteristics of the basic substrate-metallic film combination in a number of different ways, to a desired extent and in a simple and time-saving manner.
Another object of the invention is to provide a method which renders it possible to convert a basic foil consisting of a substrate and one or more metallic films into a superior foil in a simple, inexpensive and timesaving way.
An additional object of the invention is to provide an apparatus for the practice of the above outlined method.
One feature of the invention resides in the provision of a foil which comprises a substrate having a first side and a second side, a metallic film which adheres to at least one side of the substrate and has a surface facing away from the one side of the substrate, and a protective layer which overlies and adheres to the entire surface of the film. The layer preferably contains or consists of an organic material which strongly adheres to the surface of the film. Such organic material can have a molecular weight of at least about 10,000 g/mol or a molecular weight of less than about 10,000 g/mol. The organic material can be selected from the group consisting of natural and synthetic resins and natural and synthetic waxes and lubricants. For example, the organic material can contain a resin for lacquers and varnishes, a non-smearing wax or caoutchouc. Furthermore, the organic material can constitute a priming which facilitates the application of printed matter to the protective layer. Even furthermore, the layer is substantially no-smearing.
The layer is preferably thin, most preferably very thin. For example, the layer can have a thickness of about 0.5xcexc to about {fraction (1/15,000)} mm. Such layer is preferably applied to the metallic film while the latter is still devoid of scratches and/or other defects which would render the film permeable to gases. For example, the metallic material of the film can be vaporized onto the one side of the substrate and the organic material of the protective layer can be vaporized onto successive increments of the freshly vaporized film. A metal film is generally considered free of scratches if the film does not have uneven areas which are noticeable in light transmission. For example, in transmission, a foil which has pin windows is covered with small illuminated dots. The lower the number of such mechanical defects in the metal film, the less pervious the foil is to gases. A protective coating made of an organic material which is applied immediately after coating the metal film and which has a high affinity to the metal, reduces the permeability of the foil to gases to {fraction (1/10)} of that of an unprotected metal film.
The protective layer and the film can jointly constitute a fluidtight coating at the one side of the substrate. Furthermore, the layer can be resistant to corrosion, i.e., it can prevent oxidation of the metallic film.
The layer is preferably made in such a way that its thickness is constant along the entire surface of the film. Furthermore, the protective layer can be resistant to blocking. The organic material of the layer preferably exhibits high affinity for the metallic material of the film, and the layer is preferably made of an imprintable material.
The foil can further include a coating which overlies and adheres to the protective layer. The coating can constitute an extrusion which is crystallized on and uniformly covers the layer.
It is further desirable to make the layer of a physiologically acceptable material and to utilize an odorless material.
The substrate can be made of a material which is selected from the group consisting of polypropylene, polyethylene, polyesters, polyamides, polystyrene and polyvinyl chloride.
The preferably thin layer is or can be made of a material which has a low coefficient of friction, i.e., which has a smooth exposed surface and can move relative to an abutting surface in response to the application of a relatively small force. The term xe2x80x9cprotectivexe2x80x9d is intended to denote, among others, that the layer can shield the surface of the metallic film from scuffing.
Another feature of the invention resides in the provision of a method of making a foil of the above outlined character. The method comprises the steps of applying a film of metallic material (e.g., aluminum) to at least one side of a substrate so that the film has a surface which faces away from the substrate, and bonding a protective layer to the surface of the film. The layer can contain an organic material, and the bonding step can include vaporizing the organic material onto the surface of the film.
The method can further comprise the step of convoluting the foil upon completion of the bonding step. The applying step of such method can include vaporizing the metallic material onto the at least one side of the substrate, and the bonding step can include vaporizing the organic material onto the film immediately following the applying step. The organic material can be selected from the group consisting of natural and synthetic resins, and natural and synthetic waxes and lubricants. The film is ready to withstand a force which is applied to it by way of the protective layer as soon as the making of the layer is completed.
The protective layer is preferably thin and its thickness is preferably constant along the entire surface of the film.
The applying step can include vaporizing the metallic material onto the at least one side of the substrate in a vaporizer, and the bonding step can include vaporizing the organic material of the layer onto the surface of the film while the respective portion of the substrate is still in the vaporizer. The organic material of the layer is preferably selected in such a way that it exhibits high affinity for the metallic material of the film.
The high affinity between the plastic material in the vapor phase and the metal in the metal film is due to molecular interactions. After the metal is evaporated on the plastic foil in the form of a metal film, a relatively large surface tension develops in the metal film which can cause the metal film to react strongly to mechanical stress. When the foil with the applied metal film moves across a deflection roller, so-called xe2x80x9cpin windowsxe2x80x9d can form in the highly stressed surface of the metal film which destroy the initial imperviousness of the metal film to gases.
If a protective coating which has a high affinity to the metal film, is evaporated onto the metal film immediately after coating the metal film, then the large surface tension in the metal of the metal film is absorbed due to the high affinity. The metal film then becomes less susceptive to mechanical stress and tends to produce a smaller number of pin windows. The foil onto which the metal film is evaporated, can then be deflected over rollers and wound up without adversely affecting the imperviousness of the metal. The mechanical strength of a 1.5 xcexcm thick metal film is approximately 2 Newton (N) which is sufficient so that other materials can be laminated or imprinted on the foil after a protective coating is evaporated onto the metal film.
It has not yet been investigated in detail what causes an organic material in the vapor state to have a high affinity to the metal. An extensive array of experiments with potentially useful resins and waxes were conducted and a selection among waxes with a potential for forming protective layers were found. Thus, it was feasible to select organic materials which are capable of reducing the surface tension in the applied metal film. Examined are primarily so-called polar resins which have a stronger adhesion to the metal of the metal film than non-polar resins. It is recommended to test each specific application to find suitable materials which have a high affinity to the metal and which also meet other requirements, such as lamination and imprinting. The greater the required adhesion, the higher the required affinity of the resin to the evaporated metal, and the more careful the selection has to be made. Because the plastic coating is extremely thin, it has excellent durability. The very thin plastic coating also lowers the surface tension, so that the metal surface is more resistant to mechanical stress and does not form very small pin windows (1.4 xcexcm).
If the layer contains a resin, the bonding step can include heating the resin to vaporization temperature in a vaporizer and contacting the surface of the film with vaporized resin in the vaporizer.
The bonding step can include a plurality of successive stages. The method then preferably further comprises the step of advancing the substrate and the film thereon along a predetermined path, and each stage can include contacting the film with vaporized organic material in successive portions of the path. The contacting steps can include applying a discrete stratum of organic material to the film in each portion of the path so that the application of a next-following stratum begins immediately following completion of application of a preceding stratum. Such method preferably comprises the step of solidifying successively applied strata of the layer, and the contacting steps can include applying each next-following stratum subsequent to start of solidification of the immediately preceding stratum.
Successive stages of the bonding step can be carried out at predetermined intervals, preferably at intervals of 30 to 120 seconds. The duration of the intervals can depend upon the characteristics of the corresponding strata; alternatively, such intervals can be fixed or variable. The arrangement may be such that the duration of intervals increases or decreases from each preceding stage to the next-following stage of the plurality of successive stages. Each stage can involve the application of one and the same material to the film; alternatively, at least one stage can involve the application of a first material and at least one other stage can involve the application of a different second material to the film. For example, one of the materials can be a resin and another material can be a wax. If the stages include three or more successive stages, the bonding step can include alternatingly applying resin and wax during successive stages of such plurality of successive stages.
It is also important that the plastic material is non-smearing. A plastic material is basically non-smearing if the mechanical strength at temperatures to which the packing materials are subjected (xe2x88x9220xc2x0 C. to +50xc2x0 C.) is such that the plastic material is not abraded from the surface of the metal film when certain forces are applied. The plastic material adheres immovably to the metal film and does not become detached even in situations where a material which transmits large frictional forces to the protective layer, is rubbed across the protective layer, for example a rubber sponge.
The layer of the foil should also be non-blocking. Non-blocking is attained when the protective coating does not detach from the metal film even when the foil is heavily stressed, for example by repeated buckling or flexing. The tendency for the protective coating to become detached increases with the coating thickness. A thin layer with a coating thickness of {fraction (1/15000)} mm is not expected to detach from the metal film even under high stress. Quite important is also the surface quality of the plastic foil to which the metal film is applied. The smoothness of the metal film coated on the foil and the adhesion between the metal film and the plastic coating containing the plastic material with a high affinity to the metal, directly depends on the smoothness of the foil. For example, the polyethylene foil used by the applicant does not block the protective coating. A polypropylene foil provides a blocking affect which is only marginally superior to that provided by the PE foil. The gas-tight foils prepared by the applicant have a negligible blocking effect close to 0.
Also, plastic material used in such application should be xe2x80x9charmlessxe2x80x9d for humans. Harmless materials are such material that are used as a packaging material according to the BGA regulations in Germany and the FDA regulations in the USA. Plastic materials which fall within these regulatory limits, are selected as suitable plastic materials for forming the protective coating. The regulatory terms mandate that these protective coatings must not be capable of adversely affecting packaged food so as to cause illnesses in people, for example cancer.
A presently preferred embodiment of the method further comprises the step of advancing the substrate along a predetermined path in a predetermined direction and at a predetermined speed, and the applying step includes vaporizing the metallic material onto the at least one side of the advancing substrate at the predetermined speed and while the substrate advances along a first portion of its path. The bonding step comprises establishing a supply of organic material adjacent a second portion of the path downstream of the first portion, heating the supply to vaporization temperature, and contacting the film with vaporized organic material in the second portion of the path. The heating step can include electrically heating the supply of organic material.